1. Field of the Technology
The present technology relates to a full-color image forming apparatus using an electrophotographic method.
2. Description of the Related Art
Although an image forming apparatus using an electrophotographic method which was at first developed was of a type capable of forming only a monochrome image (a black- and white image), there has been developed another type of image forming apparatus capable of forming a full-color image, namely, a full-color image forming apparatus with diversification of images expected to be formed, and which has been widely used.
However images to be formed by an image forming apparatus have diversified, it is the present condition that, even in a full-color image forming apparatus, the frequency of forming monochrome images is higher than the frequency of forming full-color images, and the frequency of forming monochrome images is about five times that of forming full-color images.
Therefore, in order to form a large amount of monochrome images in a full-color image forming apparatus, different proposals to increase the efficiency of monochrome image formation are offered. For example, in Japanese Unexamined Patent Publication JP-A 5-336331 (1993), it is proposed to conduct changeover of image formation speed between a full-color image formation mode and a monochrome image formation mode, and as well set the number of scanning lines of a laser beam in the monochrome image formation mode larger than the number of scanning lines of a laser beam in the full-color image formation mode.
Further, in Japanese Unexamined Patent Publication JP-A 2000-280523, it is proposed to set the number of light beams outputted from an optical scanner at the time of monochrome printing larger than the number of light beams outputted from the optical scanner at the time of color printing. Furthermore, in Japanese Unexamined Patent Publication JP-A 2003-266781, it is proposed to make an inscribed radius of a polygon mirror for monochrome image formation, which is more frequently used in printing, smaller than an inscribed radius of a polygon mirror for color, and also increase the number of mirror planes of the polygon mirror, thereby shortening a rise time before the polygon mirror for monochrome image formation starts rotating from the stop state.
The techniques disclosed in JP-A 5-336331, JP-A 2000-280523 and JP-A 2003-266781 are effective for enhancing the efficiency of monochrome image formation. However, as mentioned before, the frequency of monochrome image formation is about five times the frequency of color image formation. Therefore, unless lengths of life of apparatus members relating to monochrome image formation are at least five times those of the apparatus members relating to color image formation, a length of life of a full-color image forming apparatus is limited by monochrome image formation. That is to say, the length of life of the full-color image forming apparatus is limited by the apparatus member relating to monochrome image formation though the length of life of the apparatus member relating to color image formation is left, with the result that replacement or repair of the apparatus member is required. JP-A 5-336331, JP-A 2000-280523 or JP-A 2003-266781 does not disclose any technique relating to extension of the length of life of the apparatus member relating to monochrome image formation or the length of life of the full-color image forming apparatus. Besides, the technique disclosed in JP-A 2003-266781 has a problem that increase of the number of the mirror planes of the polygon mirror leads to increase in costs.
As an art for seeking extension of the length of life of the apparatus member relating to monochrome image formation, which is more frequently used in the full-color image forming apparatus, in Japanese Unexamined Patent Publication JP-A 2000-242057, it is proposed to make a diameter of a photoreceptor for monochrome image formation larger than a diameter of a photoreceptor for color, and in Japanese Unexamined Patent Publication JP-A 2001-330976, it is proposed to make film thickness of a photosensitive layer of the photoreceptor for monochrome image formation more than film thickness of a photosensitive layer of the photoreceptor for color.
According to JP-A 2000-242057 and JP-A 2001-330976, mainly the amount of shaving of a photosensitive layer is critical to the length of life of the photoreceptor. Therefore, by making the diameter of the photoreceptor larger, the frequency with which a circumferential surface of the photoreceptor is shaved by a cleaning blade or the like is decreased, with the result that the length of life is extended. Moreover, by thickening the photosensitive layer itself, the length of life of the photoreceptor is extended.
The apparatus member relating to monochrome image formation is not only the photoreceptor for monochrome image formation but also a light source for monochrome image formation for exposing the photoreceptor for monochrome image formation to light corresponding to monochrome image information. Since the light source for monochrome image formation is used with the same frequency as the photoreceptor for monochrome image formation at the time of monochrome image formation, the length of life thereof gradually decreases while being used as well as the length of life of the photoreceptor for monochrome image formation. Therefore, it is impossible to realize extension of the length of life of the full-color image forming apparatus only by seeking extension of the length of life of the photoreceptor for monochrome image formation, and it is necessary to extend the length of life of the light source for monochrome image formation used with the same frequency as the photoreceptor for monochrome image formation. However, JP-A 2000-242057 or JP-A 2001-330976 does not disclose any technique for seeking the length of life of the light source for monochrome image formation.